chapter 2 multiple access protocols professor rick han university of colorado at boulder...

Chapter 2Multiple Access Protocols

Professor Rick HanUniversity of Colorado at Boulder

[email protected]

Prof. Rick Han, University of Colorado at Boulder

Announcements

• Email/Hand in HW #1 in class Thursday Jan. 29

• Programming assignment #1 is online and is due Feb. 10• Dora submission

• No TA office hours this week• Next, Chapter 2, Media Access Protocols


Recap of Previous Lecture• Stop-and-Wait• Sliding Window protocols – keep the bit

pipe full• Go-Back-N• Window-based Flow Control• Selective Repeat Protocol

• Preview of Shared-media protocols


Shared-Media or Broadcast Networks

• N senders and receivers connected by a shared medium (copper wire, atmosphere, water)

• Shared local access to the same media• Local Area Network (LAN)

• Ethernet, Fast Ethernet, Gigabit Ethernet, …• Wireless Ethernet, or 802.11 a/b/g, or WiFi

Ethernet (802.3) 802.11/WirelessEthernet


Multiple Access Protocols

• Determine which host is allowed to transmit next to a shared medium• Channel reservation: TDMA, FDMA, CDMA,

Token Ring, …• Random access: ALOHA, CSMA/CD, CSMA/CA

Ethernet 802.11/WirelessEthernet


Multiple Access Protocols (2)

• Also called Medium-Access Control (MAC) protocols

• Before data link-layer packets can be sent, a sender has to gain access to the media • MAC layer is often placed in

the stack between layer 2 and layer 1

Physical Layer

MAC Layer

Host A

Data LinkLayer


Time Division Multiple Access (TDMA)

• Divide time into multiple slots• Each host sends in a pre-determined slot• Out-of-band reservation mechanism• Compare to Time Division Multiplexing (TDM)

… …2 3 11 2 3 1 2 3

Host 1

Host 2

Host 31

2

3

Not Eth.

Router/Mux

1

2

12


Frequency Division Multiple Access (FDMA)

• Divide spectrum into frequency bins• Each host sends in a pre-determined

frequency bin• Out-of-band reservation mechanism (FCC)• Also called Frequency Division Multiplexing

(FDM)• Example: AM/FM radio, TV

AM500-1700 KHz

FM88-108 MHz

SatelliteGHz range

Freq.(Hz)

Host 1 Host 2 Host 3


Code Division Multiple Access (CDMA)

• Use multiple orthogonal codes to partition a range of spectrum

• Each host sends using a pre-determined code• Also called “spread spectrum”

• Two forms spread spectrum:• Direct-Sequence Spread Spectrum– DSSS

• Chipping sequences spread the signal’s spectrum

• CDMA is often used as synonym for DSSS• Examples: 802.11b, cell

• Frequency-hopping spread spectrum– FHSS• Example: Bluetooth• Advantage: simple, but not as efficient


Code Division Multiple Access (CDMA) (2)

• Frequency hopping example

BluetoothHost 1’s Code: 1342, Host 2’s Code: 3214, Host 3’s Code: 4123Note that all 3 codes are orthogonal: at each instant in time, each host is on a different frequency

Host 1

Host 2

Host 3

F1 F2 F3Freq (Hz)

F4

F1 F2 F3

…, F1, F3, F4, F2, F1, F3, F4, F2, …

Possiblehoppingsequence

F4


Random Access/MAC Protocols

• Multiple users share the same frequency band and/or same time and/or same code• Analogy: conversation in a crowded room

• What protocol steps do people use to talk in the same room (shared media)?• Important factors:

• Wait for silence• Then talk• Listen while talking.• What do we do if there’s 2 talkers?

Backoff.• Repeat

• Protocols also add a random increasing timeout


Random Access: ALOHA Protocol

• Developed at University of Hawaii in 1971 by Abramson• Ground-based UHF radios connect computers

on several island campuses to main university computer on Oahu

• “pure” ALOHA: hosts transmit whenever they have information to send – form of random access• Collision will occur when two hosts try to

transmit packets at the same time• Hosts wait a timeout=1 RTT for an ACK. • If no ACK by timeout, then wait a randomly

selected delay to avoid repeated collisions, then retransmit


Random Access: ALOHA Protocol (2)

• Collision of packets can occur when a packet overlaps another packet

Packet A

Packet CPacket B

time

T0

Collision Collision

Wasted TimeColliding with B

Wasted Time Due to a Collision = 2 packet intervals


Random Access: Slotted ALOHA

• Rather than sending a packet at any time, send along time slot boundaries• Collisions are confined to one time slot

Packet A

Packet CPacket B

time

T0

CollisionNo

Collision

Wasted Time Due to a Collision = 1 packet interval


Random Access: Slotted ALOHA (2)

• How do hosts synchronize to begin transmitting along time slot boundaries?• One central station transmits a

synchronization pulse or beacon

• Slotted ALOHA is more efficient than ALOHA because when there is a collision, the wasted time is confined to one time slot• Assuming Poisson packet arrivals

(memoryless), can compute the maximum throughput of ALOHA to be 18%.

• Maximum throughput of Slotted ALOHA is 37%

• Why are ALOHA & slotted ALOHA so inefficient?


Random Access: CSMA • ALOHA & slotted ALOHA are inefficient

because hosts don’t take into account what other hosts are doing before they transmit• “Talk-before-listen” protocols• Example: at party, everyone speaks whenever

they want to, regardless of whether another person is speaking

• Instead, “listen before you talk” = Carrier Sense Multiple Access (CSMA)• Sense for “carriers” (see if anyone else is

transmitting) before you begin transmittingPacket A timeHost Blistens

Packet B Packet YPacket X

Host B sendsdelay Collision still possibleover long prop. delays


Random Access: 1-Persistent CSMA

• If channel is busy,• A host listens continuously• When channel becomes free, a host transmits its

packet immediately (with probability 1)

Packet Atime

Host B listens

Packet B Packet YPacket X

Host B sends Collision

• Collision scenarios• Hosts A and B are far apart (long prop. delay).

A’s signal takes a long time to reach B. So, B thinks channel is free, and begins transmitting.

• Hosts B and C transmit as soon as A finishes• Still, CSMA is more efficient than ALOHA variants


Random Access: p-Persistent CSMA

• Generalization of 1-persistent CSMA• Typically applied to slotted channels• Slot length is chosen as maximum propagation

delay

• A host senses the channel, and• If slot is idle, transmit with probability p, or

defer with probability q=1-p• If next slot is idle, transmit with probability p,

or defer with probability 1-p, repeat…• If channel is busy, then sense channel

continuously until it becomes free, begin again


Random Access: Non-Persistent CSMA

• Host does not sense channel continuously• Instead, if channel is busy,

• Wait/sleep a random interval before sensing again

• As with 1-persistent CSMA, as soon as channel is idle, then send a packet

• Random interval reduces collisions• Higher throughput than 1-persistent CSMA

when many sendersPacket A time

Host Blistens

Packet B

Host B sendsRandomSleep


Random Access: Ethernet CSMA/CD

• Ethernet uses CSMA/CD, i.e. CSMA with Collision Detection (CD)

• “Listen-while-talk” protocol• A host listens even while it is transmitting, and

if a collision is detected, stops transmitting

Packet Atime

Host Bsensescarrier

Packet B

delay

Host B starts sending

Packet B

Host B detects collisionAnd stops sending

Not transmitted


Random Access: Ethernet CSMA/CD (2)

• Can abort transmission sooner than end-of-packet if there is a collision• Can happen if prop. delays are long• Better efficiency than pure CSMA

• CSMA/CD doesn’t require explicit acknowledgement• Unlike CSMA, which requires an ACK or timeout

to detect a collision• Collision detection is built into the transmitter• When collision detected, begin retransmission



• Exponential backoff strategy• When a collision is detected, a host waits for

some randomly chosen time, then retransmits a packet

• If a second collision is detected, a host doubles the original wait time, then retransmits the packet

• Each time there is another collision, the wait time is doubled before retransmission

• Variants:• At each retransmission, choose a random value from

the exponentially increasing wait time.• At each retransmission, choose randomly from

among a discrete set of values within exponentially increasing wait time

• Retransmit a finite # of times



• CSMA/CD can be used with nonpersistent, 1-persistent, or p-persistent variants of CSMA

• Ethernet is synonymous with the IEEE 802.3 standard• Initial work on Ethernet at Xerox in early 70’s• Ethernet specifies 1-persistent CSMA/CD

• To extend an Ethernet, repeaters are placed.• Start to run into propagation delay issues and

noise amplification issues• Ethernet keeps its maximum length to 2500 m

to keep prop. delays tight, so that CSMA/CD responds well



• Ethernet CSMA/CD requires a minimum size to a frame:

A BEthernet

Hosts A and B at opposite ends of the Ethernet

t t+d, d=prop. delay

B transmits @ time t+dJust before A’s packet arrives

B sees Collision at t+d,Transmits a “runt” packetB’s packet arrives at t+2d



• If B’s packet arrives at A and A is no longer transmitting, then Host A will• Fail to detect the collision• thinks its packet got through• Thinks the incoming packet is a new packet

• Therefore, to detect a collision:• Minimum frame size >= 2*(prop. delay)*BW

A BEthernet

Hosts A & B at opposite ends of Ethernet

t t+d

B’s packet arrives at t+2d


Random Access: 802.11 “Wireless” Ethernet

• Employs CSMA/CA, i.e. CSMA with Collision Avoidance (CA)

• Hidden terminal effect• Example: B can hear A and C, but A and C

can’t hear each other. If A is sending B, C thinks channel is clear and starts sending => collision!

• Doesn’t happen in wired Ethernet, because hosts can hear each otherHost A

Collision

Host B Host C


Random Access: 802.11 “Wireless” Ethernet (2)

• How to handle the hidden terminal effect?• Host A sends a Request-To-Send (RTS)• Host B sends a Clear-To-Send (CTS)• Host C hears the CTS, and does not interrupt

transmission between A and B• This helps implement Collision Avoidance

Host A

Host C Suppresses Its Data

Host B Host CRTS

CTS CTS

Data

ACK

chapter 2 multiple access protocols professor rick han university of colorado at boulder...

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